1. Field of the Disclosure
The present disclosure relates to a waveguide. In particular, aspects of the present disclosure relates to a waveguide for use in a sensor system for monitoring the clearance between a moving member such as a rotary blade and a casing structure within which the moving member is moving e.g. a turbine casing of a gas turbine engine.
2. Description of the Related Art
It is desirable to monitor the clearance between the tip of a rotary blade and a casing of a gas turbine engine to monitor positional/dimensional shifts in turbine components and to provide feedback for tip clearance control (TCC) systems which regulate expansion or contraction of the turbine casing (which affects the clearance between the rotary blade tips and the turbine casing).
A system for measuring/monitoring the clearance between a moving member such as a rotary/aerofoil blade and casing structure e.g. a turbine casing is described in US2011/0194122. Such systems generate a microwave signal which is directed in a radially inwards direction through a waveguide extending through the turbine casing. Portions of the signal are reflected radially outwards through the waveguide by a reference feature within the waveguide, by the casing surface and by the tips of the aerofoil blades as they move (rotate) within the turbine casing. The reflected signal portions are processed using standard signal distance measuring techniques such as interferometry or time of flight measurement which use the relative phase and/or timing of the signal portions to determine the relative positions of the various elements.
The waveguide used in this known system is typically a single-piece, metallic element that is inserted through the turbine casing after assembly of the turbine. Once fitted, it may be a permanent feature or it may be replaceable.
A turbine casing typically comprises a radially outer casing portion formed of forged steel or nickel alloys for withstanding the internal gas pressures within the turbine. This outer casing is lined with seal segments which seal the rotor path and, in some cases, form or are provided with an abradable layer into which the rotary blades can cut to form a circumferentially extending channel with a view to minimising tip clearance. The seal segments are usually but not exclusively mounted onto the outer casing by segment carriers.
A problem with this known system is that the waveguide is rigid and thus axial movement between the outer casing, seal segments and segment carriers of the turbine casing cannot be accommodated. Furthermore, since the waveguide is inserted through the turbine casing after manufacture, sealing around the waveguide is problematic and effective sealing of the gas flow path to prevent leakage of gas/heat through the turbine casing can be difficult to achieve.